The present disclosure relates to an image forming apparatus including a charging member which charges an image carrier, and in particular relates to a method for appropriately controlling a peak-to-peak voltage value of an alternating-current voltage applied to the charging member.
In conventional image forming apparatuses using an electro-photographic process, such as laser printers and digital multifunction peripherals, the following process is typically performed. A surface of a photosensitive drum (an image carrier) having photoconductivity is uniformly charged by a charging device, then the surface of the photosensitive drum is exposed to light from an exposure device to form an electrostatic latent image on the photosensitive drum, and then the thus formed electrostatic latent image is developed into a toner image by a developing device. Next, after this toner image is transferred onto a surface of a recording medium such as a sheet by a transfer section, the toner image is fixed by a fixing section onto the surface of the recording medium, and this completes a process of a series of image formation. After the transfer of the toner image, residual toner remaining on the surface of the photosensitive drum is removed by a cleaning section, and further, residual charge remaining on the surface of the photosensitive drum is removed as necessary by using a charge removing lamp, whereby the photosensitive drum is made ready for the next image formation.
In recent years, instead of corotron-type and scorotron-type charging devices, a contact charging type charging device with little generation of ozone is used, in which the charging member (a charging roller or the like) is disposed in contact with, or close to, the photosensitive drum to charge the photosensitive drum. Among this type of charging members, there is one to which is applied an oscillation voltage, in which a direct-current (DC) voltage and an alternating-current (AC) voltage are superimposed, to charge the photosensitive drum.
For example, it is known that, when a peak-to-peak voltage Vpp of the AC voltage in the oscillation voltage is raised, a charging voltage of the photosensitive drum rises in proportion to the rise of the peak-to-voltage Vpp, and a charging potential is saturated when the peak-to-peak voltage Vpp reaches a level approximately twice the level of a charging start voltage of the DC voltage, such that the charging potential does not vary much even if the peak-to-peak voltage Vpp is further raised. It is also known that, for securely uniform charging, it is necessary for the peak-to-peak voltage Vpp of the applied oscillation voltage to be equal to, or higher than, twice the charging start voltage in applying the DC voltage determined by various characteristics of the image carrier, and that the charging voltage obtained at that time depends on a DC component of the applied voltage.
There is also known one capable of setting a highly accurate appropriate peak-to-peak voltage Vpp of an AC voltage regardless of change in ambient conditions such as temperature and humidity or regardless of aging of the photosensitive drum, the charging member, and the like. Specifically, for the purpose of obtaining an appropriate peak-to-peak voltage value, an appropriate charging start voltage is calculated from two peak-to-peak voltages lower than twice a charging start voltage and one peak-to-peak voltage equal to, or higher than, twice the charging start voltage, and the calculated appropriate charging start voltage is maintained constant as the peak-to-peak voltage of an AC voltage applied to a charging member in forming an image.